In the background of increasing automobiles and frequent traffic jams, recently, so-called navigation system is drawing wide attentions (for example, Japanese Laid-open Patent 10300/1984, Japanese Laid-open Utility Model 24200/1992, which is incorporated herein by reference), and in particular the route searching apparatus is noticed together with the position measuring apparatus.
In the conventional route searching system, when the departure point and destination point are entered, first, the range to be searched is determined from the information of the two locations. In consequence, all nodes existing in this searching range are read out from the map information memory means, and the shortest route between the two points is found. Herein, the node is defined to be the flex point entered for the purpose of convenience in the map information in order to express the intersection or curve of the roads.
The time required for calculation of the shortest distance is proportional to the square of the number of nodes within the searching range. Therefore, the longer the distance between the departure point and the destination point, the longer time is taken for calculation, and a memory of large capacity is needed for calculation. Still more, since the system is mainly designed to deduce the shortest distance, it is highly possible that the result of search contrary to the expectation of the driver - wishing to drive wide highway, avoiding narrow local road --may be produced, and the obtained routes were often useless.
According to the Japanese Laid-open Utility Model 85000/1990, which is incorporated herewin by reference, nodes existing in a range of a specific width around the line linking the departure points and destination points are selected, and links containing broader roads are selected from the nodes existing in this range, and therefore routes comprising broader roads are searched.
In this method, if the distance between the departure point and destination point is very long, nodes are selected by limiting to a range of a specific width (.DELTA.D). Therefore, if there is a broad trunk line at a position slightly remote from this range, the road is out of the searching range, and is not displayed as shortest route and hence not used.